1. Field of the Invention
The present invention relates to disc rotating apparatuses. More particularly, the invention relates to a structure that corrects for wow and flutter or rotation deflection caused due to deviation in position between a spindle motor rotation axis about which the turntable rotates at high speeds and a center of gravity of a disc being mounted on the turntable (may be hereinafter referred to as an "unbalanced state" or "deviation in center of gravity"), for a disc player to reproduce, record or erase information memorized on a disc recording medium (hereinafter merely referred to as a "disc") such as an optical disc, magnet-optical disc or the like.
2. Description of the Prior Art
If such a turntable as above is rotated at a high speed in a state of unbalance or position deviation in center of gravity, the turntable becomes vibrating thereby causing reproduction error and the like. In order to prevent this, there conventionally has been a rating that the deviation in center of gravity be specified to 1 g. cm or less.
Under such a situation, various proposals have been made in order to reduce wow and flutter or rotation deflection caused due to deviation in center of gravity. For example, Japanese Patent Laying-open No. S51-73318 discloses a first prior art. This prior art has a sensor arranged on the turntable to detect a deviated center of gravity, and a movement adjusting mechanism using a bimetal member arranged under the turntable so that the movement adjusting mechanism can be controlled by a signal supplied from the sensor.
In this first prior art, however, the sensor and the movement adjusting mechanism have to be arranged at a location close to the turntable, making the structure complicate. Furthermore, the movement adjusting mechanism requires a means for controlling it, resulting in inexpensive cost.
As a second prior art, there is a technology as disclosed, for example, in Japanese Patent Laying-open No. S63-15079 and Japanese Patent Laying-open No. H2-139758. According to this technology, an annular liquid tank is provided in the turntable so that an unbalanced state can be corrected for by a liquid filled within the liquid tank. This second prior art requires to enclose a liquid within the liquid tank. However, this enclosure is complicate in structure hence requiring expensive cost.
Meanwhile, there is a third prior art depicted in FIG. 1. The disc player 1 of the prior art in FIG. 1 includes a turntable 2 firmly fixed on an rotary shaft 3a of a spindle motor 3. Around an outer periphery of the turntable 2 a swing frame 4 is fitted, and a disc holding frame 5 is supported by the swing frame 4. The holding frame 5 has a frame main body 5a almost in the same diameter as that of the turntable 2, and a lid 5b fitted over a top surface of the frame main body 5a. The frame main body 5a has an opening 5c formed at a center thereof. The lid 5b and the frame main body 5a define therebetween a space, at a central location of which a ring magnet 6 is placed. This ring magnet 6 is fitted over an outer periphery of a projection 7 formed at almost a center of the lid 5b. The turntable 2 is formed, at its center, with a projection 2a having an annular recess formed therein so that a magnetic ring 8, for example, of an iron or the like is placed in the annular recess.
In the FIG. 1 prior art, a disc D is mounted on the turntable 2 in a state that the disc holding frame 5 is positioned distant from the turntable 2. By lowering the swing frame 4, a magnetic attractive force is acted upon between the ring magnet 6 and the magnetic ring 8 so that the holding frame 5 can hold the disc D onto the turntable 2. Due to this, the disc D is clamped between the holding frame 5 and the turntable 2. The spindle motor 3 is then driven to rotate the disc D through the turntable 2 at a high speed, and an optical pickup (not shown) is actuated. This makes it possible to reproduce the information memorized on the disc D, record information onto the disc D, or erase information memorized on the disc D.
In this prior art, an annular groove 9 is formed, about a center axis O of the rotary shaft 3a, at a location between the magnet ring 6 and the frame main body 5a of the disc holding frame 5 as shown in FIGS. 1 and 2, in order to prevent against an unbalanced state as stated before. A plurality of steel balls 9a for unbalance correction are distributed in the annular groove 9 so that the steel balls 9a can freely move in a circumferential direction of the annular groove 9. That is, the annular groove 9 has its width H somewhat greater than a diameter of the steel ball 9a so that the steel ball 9a can freely move in the circumferential direction of the annular groove 9.
With this structure, when an unbalanced state occurs, the steel balls 9a are moved to a position opposite to a center of gravity G of the disc D with respect to the axis O as shown by a virtual line in FIG. 2 by a centrifugal force due to high speed rotation of the turntable 2, thereby eliminating the unbalanced state.
In the prior art shown in FIGS. 1 and 2, however, all the steel balls 9a are moved to a position that is opposite to the center of gravity G and is an outer side of an inner wall of the annular groove 9. Consequently, unbalance correction is made by a constant amount for the turntable 2 in a fashion irrespective of a distance L of deviation of the center of the gravity G from the axis O. This, however, might cause over-correction as the case may be.
Moreover, in the prior arts of FIGS. 1 and 2, even where there exists almost no unbalanced state or deviation in center of gravity wherein the center of gravity G of the disc D and the axis O are coincident in position with each other, the steel balls 9a gathering together are magnetized by the magnet ring 6 and attracted to each other, as shown by the virtual line in FIG. 2. Thus there has been a problem that excessive increase in mass is encountered at around a portion where the steel balls 9a gather, rather giving rise to an unbalanced state.